1. Field of the Invention
The invention relates to methods of introducing porous membranes into microelectromechanical (MEMS) elements by supporting the membranes by frames.
2. Description of the Prior Art
MEMS membranes have not yet been made by sintering granular matter, but by etching holes into a previously formed membrane, e.g. out of KOH-etching in silicon. Also polymer layers like PTFE containing pores have been incorporated into stacked systems. This, however requires hybrid mounting techniques.
Besides these techniques anodized alumina is available, since during electrochemical oxidation of an aluminum sheets vertical, parallel ordered pores of alumina with diameters in the range of 100 nm and a length of several tens of microns are formed. By dissolving the remaining aluminum and opening the pore bottoms, a highly porous membrane is formed. Single sheets of porous alumina have been already made available for filtering purposes and some attempts have been reported to use these porous layers as substrates for magnetic data storage or photonic crystals.
Some other materials have been reported as being fabricated in a porous form, e.g. silicon and indium phosphide, but no fabrication of membranes out of these materials has been reported.
Since alumina is a very hard and brittle material, membranes made of it could withstand high differential pressures, but they will break easily during handling. Ideas of making monolithically supported alumina membranes e.g. on silicon or Pyrex are hindered by the erosion of aluminum and porous alumina during substrate etches as well as, reversely, erosion of some substrates and resists during long anodizations in most of the acids. Additionally, quite thick aluminum layers (ranging from a few to tens of microns) are necessary to achieve a well-ordered porosity. This thickness cannot be achieved by typical vapor deposition methods of the metal. Thus, it is difficult to incorporate formation of porous alumina layers by processing alumina layers in a MEMS or CMOS process.
What is needed is some type of method to electrochemically make porous membranes which are mechanically stable in all processing, handling and mounting steps usually performed in the fabrication of microelectromechanical or microfluidic systems.
The invention is a method of introducing porous membranes into MEMS elements by supporting the membranes by frames to form an heterostructure. This is achieved by attaching to a structured or porous substrate one or more monolithically fabricated frames and membranes. Having membranes disposed on frames enables them to be batch processed and facilitates separation, handling and mounting within MEMS or nanofluidic systems. Applications include, but are not limited to, filters for gases or liquids, electron transmissive windows and scanning electron microscopy (SEM) accessible arrays of nanotest tubes containing liquid phases and other sample states.
More specifically, the invention is defined as a method of fabricating a porous membrane comprising the steps of fabricating a porous membrane in a preporous first substrate in which the porous membrane has, an initial thickness which is greater than a predetermined final thickness. The porous membrane is then attached to a second substrate. The porous membrane is then thinned down to the thinner predetermined final thickness. The invention may also be practiced by omitting the thinning step.
The method further comprises selectively opening areas in the second substrate to define a frame to which the porous membrane is attached. The open areas in the second substrate are temporarily filled prior to thinning the porous membrane. In the illustrated embodiment, the step of thinning the porous membrane comprises grinding the porous membrane down to thinner predetermined final thickness.
Where the porous layer is composed of alumina, the step of attaching the porous membrane to a substrate comprises attaching the porous membrane to the second substrate by means of an adhesive layer or by means of annealing a aluminum layer disposed therebetween.
In another embodiment the method further comprises disposing a sacrificial layer between the porous membrane and the second substrate. The step of fabricating the porous membrane in the preporous first substrate comprises electroplating the first substrate onto the sacrificial layer and forming pores in the electroplated first substrate. In one version of this embodiment the step of disposing a sacrificial layer comprises disposing an organometallic or sputtered seed layer as the sacrificial layer. In the case where the second substrate is composed of silicon or Pyrex, the step of disposing the sacrificial layer on the second substrate comprises disposing a Si3N4, SiC, or PTFE/Teflon layer on the second substrate.
In still another embodiment the second substrate comprises an integral nonporous region of the first substrate and the step of attaching the porous membrane to the second substrate comprises fabricating the porous membrane in the first substrate outside of the integral nonporous region of the first substrate. The nonporous region of the first substrate is selectively structured to form a frame supporting the porous membrane.
Where the porous membrane is composed of alumina, the method further comprises chemomechanical polishing the porous membrane with sulfuric acid. Where the integral nonporous region of the first substrate is composed of aluminum, the method further comprises electrochemically etching a backside of the integral nonporous region with HCI acid.
The porous membrane is characterized by pores each having a pore bottom and the method further comprises thinning or opening the pore bottoms by ion beam etching. The opened porous membrane can be employed as a filter. In this case the method further comprises filling the pores with a filtering agent and employing the opened porous membrane as a mechanically stabilized filter. Alternatively the opened porous membrane can be employed as an array of electron transmissive windows. A charge applied to the opened porous membrane then allows the porous membrane to be employed as an electron collimator. The opened porous membrane can also be employed as an array of electron beam accessible nanotest tubes. The array of electron beam accessible array nanotest tubes can be used to separate a vacuum from a liquid phase. Still further the opened porous membrane can be employed as a microextractor. The invention is thus to be understood not only as defined as the described method, but also the apparatus made by the method. It must also be expressly understood that the described applications are but a small sample of the uses to which the method of the invention and the apparatus formed by the method can be employed.
While the method has been described for the sake of grammatical fluidity as steps, it is to be expressly understood that the claims are not to be construed as limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations under 35 USC 112, but to be accorded the full scope of the meaning and equivalents of the definition provided by the claims. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.